"a treaty and agreement betwixt the commissioners for the united colonies of new england on the one part and . . [the] sagamores of the narraganset and niantic indians on the other part made and concluded at boston in the massachusetts the xxviith of the sixth month 1645." this passage suggests that sagamores were:

Leaders--they with commissioners for NE created a treaty and therefore the sagamores are spokespersons or leaders of the tribes. 

Sagamores of the tribes were meeting with NE leadership to create a treaty. Only leadership would be recognized for the treaty process. The sagamores were either leaders of the tribes or acting as diplomats to complete the treaty process.